1. Field of the Invention
The present invention relates to a method of focusing an optical head on an object body and an automatic focusing device for an optical inspection system, and more particularly, to a method and a device employable in an optical inspection system of semiconductor wafers and the like.
2. Description of Prior Arts
As well known in the field of semiconductor electronics, semiconductor devices having semiconductor integrated circuits therein are obtained through fabrication of impurity diffused regions, insulating films, wiring patterns and the like in a major surface of a semiconductor wafer. These electronic structure thus fabricated are then inspected with an inspection system in order to determine whether they are accurately formed on the wafer or not. Various non-contact inspection systems have been developed and employed for the inspection.
Within conventional non-contact inspection systems, optical inspection systems are useful ones. In an optical inspection system, an optical inspection of a wafer surface, such as measurement of a film thickness, a measurement of a line width and a check of a surface flaw, is performed with an architecture similar to that of a microscope, where focusing on a wafer surface is required. A device for automatic focusing is disclosed in Japanese Utility Model Laid Open Gazette No. 64-19909 (1989), for example.
In such an optical inspection system, required are not only a position control to adjust an inspected region on a wafer surface to a focal position of an objective lens but also an angular control or attitude control to hold the region-to-be-inspected on the wafer surface always perpendicular to the optical axis of the objective lens. Herein, the region-to-be-inspected is a minute spot of usually about several .mu.m to 50 .mu.m in diameter. In a conventional focusing device, however, a system for the angular control is provided separate from a system for a position control, and thus, the number of parts in the device is considerably large. As a result, a process of assembling and regulating the device becomes complicated, and the size of the device should be large. Moreover, the positional deviation between the position control system and the angular control system may cause reduction of accuracy in control.
Furthermore, the two kinds of control system are independently operated, and hence, there arises the problem that a control for quickly converging the position and angle in the optical system to focal ones through systematical combination of the positional control and the angular control cannot be attained.